1. Field of the Invention
This invention relates generally to a diamond wafer and a surface acoustic wave (SAW) device produced on the diamond wafer. This invention relates more specially to a method of estimating diamond wafers which judges whether a diamond wafer is suitable for producing SAW devices or not. The estimation selects diamond wafers that have a low density of surface defects which enables manufacturers to produce a low loss SAW device. The diamond wafers can also be used as the substrates of microelectronic devices with microscopic wiring or the substrates of micromachines with microscopic structures.
This application claims the priority of Japanese Patent Application No.9-290262 (290262/97) filed Oct. 6, 1997 which is incorporated herein by reference.
2. Description of Related Art
Diamond enjoys the highest sound velocity among all the natural materials. The hardness is also the highest. The thermal conductivity is large. The band gap of diamond is 5.5 eV which is an extremely high value among all known materials. Diamond is excellent in dynamical property, electrical property and electronic property. Diamond is used for dynamic devices and electronic devices that can take advantage of its excellent properties. Improvements have been made by taking advantage of the outstanding properties of diamond in the technical fields of acoustics, optics and semiconductor. Exploitation of diamond will be effective for improving various properties of electronic, acoustic devices or for enlarging margins of operation of the devices.
A surface acoustic wave device (SAW device) is a good candidate for the use of diamond for improving its characteristics. Surface acoustic wave devices can be configured to be, for example, a radio frequency filter, a phase shifter, a convolver an amplifier, etc. The SAW filter acts as an IF (intermediate frequency) filter of television sets or various filters of communication devices. A surface acoustic wave device is a device having a rigid base, a piezoelectric film stuck to the rigid base and interdigital transducers formed on both ends of the piezoelectric film. Application of an AC voltage on the interdigital transducer causes an AC electric field on the piezoelectric film which deforms in proportion to the electric field. Since the electric field oscillates, the piezoelectric film alternatively expands and contracts in the horizontal direction between two interdigital transducers at the same frequency as the AC voltage.
The piezoelectric film oscillates in the horizontal direction with the frequency of the AC signal. Since the piezoelectric film adheres to the rigid base, the rigid base also oscillates at the same frequency in the same manner. Since the rigid base repeats expansion and contraction at the interdigital transducer (IDT), the oscillation propagates as a longitudinal elastic wave on the surface. The AC voltage applied on the interdigital transducer generates an elastic wave. The wavelength is determined by the period of the interdigital transducer. The elastic wave spreads from one interdigital transducer to the other interdigital transducer. The piezoelectricity is reversible. At the receiving interdigital transducer, the deformation oscillation induces an AC voltage between the components of the electrode. As a whole, the AC signal propagates from one interdigital transducer to the other interdigital transducer by the elastic wave. The wave is called surface acoustic wave (SAW), because it propagates on the surface of the device.
The period of the interdigital transducer uniquely determines the wavelength xcex of the surface acoustic wave. The rigidity and density of the rigid base determine the velocity v of the SAW. The more rigid and lighter base brings about the higher SAW velocity. SAW velocity v is different from sound velocity which is equal to a square root of Young""s modulus E divided by density xcfx81. As with sound velocity, the SAW velocity is higher for the rigid base of higher Young""s modulus and lower density. A sound wave is an elastic wave passing through an inner portion of a material. SAW is another elastic wave propagating only on the surface of the material. SAW differs from sound wave. SAW velocity is, in general, higher than sound velocity.
Since the wavelength xcex and the velocity v have been predetermined by the interdigital transducer and the physical property of the rigid base, the frequency f is also surely determined as f=v/xcex. This is a unique value. Since f is a unique value, it is denoted by f0. Namely, the SAW device has a filtering function which selectively allows only the SAW of f0 to pass the device. SAWs of frequency different from f0 attenuate. Transmittable SAW has a definite frequency f0 which is determined by the material of the rigid base and the spatial period of the interdigital transducers. SAW devices have been applied to TV filters having a low allowable frequency of several megahertzs to tens of megahertzs. Hopefully, SAW devices will be applied to far higher frequency filters, for instance, optical communication filters of 2.488 GHz or wireless LAN filters in near future.
Raising frequency f0 requires either narrowing a spatial period of interdigital transducers or increasing a SAW velocity v. The spatial period of interdigital transducers is limited by the current lithography technology. The only way is the increase of velocity v. Diamond, as a rigid base, exhibits the highest SAW velocity among all natural materials. The application of diamond to SAW devices attracts attention. Diamond endows the SAW devices with the highest velocity which affords a moderately wide spatial period to the interdigital transducers.
High velocity is not the only requisites for a material used for SAW devices. Low propagation loss is another important requirement for SAW materials. Loss is a key concept of the present invention. There are different losses in addition to the propagation loss. Losses are now briefly clarified. One is the Joule loss xcex94Er by the resistance of the interdigital transducers to which electric power is supplied. Another is an electromechanical conversion loss 66 Ec of energy accompanying the expansion and contraction of the piezoelectric material by the AC electric field. The loss depends on the electromechanical coefficient of the piezoelectric film. The interdigital transducer which converts electric power into mechanical power through the piezoelectric film has no selectivity of direction. The surface acoustic waves propagate in both directions perpendicular to the stripes of the interdigital transducers. Just half of the mechanical power spreads toward the counterpart interdigital transducer. Another half (6 dB) is a loss. This is called a bisection loss xcex94Eb. Now a SAW starts from one interdigital transducer and some of the SAW arrives at the other interdigital transducer. The difference between the starting SAW power and arriving SAW power is the propagation loss xcex94Ep. The aim of the present invention is a reduction of the propagation loss xcex94Ep. At the other interdigital transducer, the piezoelectric film converts the mechanical power of SAW into electric power of AC voltage with a conversion loss xcex94Ec. The current flows in the receiving interdigital transducer with a resistance loss xcex94Er.
Total loss is a sum, 2xcex94Er+2xcex94Ec+xcex94Eb+xcex94Ep, of the resistance loss 2xcex94Er, the conversion loss 2xcex94Ec, the bisection loss xcex94Eb and the propagation loss xcex94Ep. xcex94Er is contingent upon interdigital transducers. xcex94Ec is ruled by the piezoelectric material. Geometry decides xcex94Eb. Only the propagation loss xcex94Ep depends upon the insulating material (rigid base). This invention aims at alleviating xcex94Ep.
The insulator which has been most widely used as the material of the rigid base is glass. Glass is an inexpensive and low-loss insulator. ZnO/glass SAW filters have been popularly employed as TV intermediate frequency filters. Zinc oxide (ZnO) is a piezoelectric material. ZnO/glass means a SAW filter having a glass substrate and a ZnO film deposited on the glass. In spite of low-loss and low-cost, glass SAW devices cannot raise operation frequency f0 owing to the low SAW velocity v. Somebody has proposed new SAW devices having a harder rigid base than glass, for example, sapphire, quartz, LiNbO3 and so on. These new materials give higher SAW velocity than glass owing to high Young""s modulus. However, the SAW velocities of sapphire SAW devices, quartz SAW devices or LiNbO3 SAW devices are still unsatisfactory for high frequency filters. Diamond is the most promising candidate which gives the highest SAW velocity due to the extreme rigidity.
Diamond SAW devices enable the current lithography technology to pattern interdigital transducers for a frequency higher than 1 GHz owing to the high SAW velocity v. However, the propagation loss xcex94Ep is still large in diamond SAW devices. The large propagation loss xcex94Ep leaves diamond SAW devices impractical. It is difficult to make a wide, flat, even, smooth and defect-free diamond film covering the whole of the device due to the extreme rigidity of diamond. It is further difficult to make a flat piezoelectric film on the defect-rich diamond film. Even if the piezoelectric film is produced, it is still a piezoelectric film of poor quality. Many defects on the diamond surface and the shortness of the SAW wavelength raise the propagation loss xcex94Ep and leave diamond SAW devices inoperative.
Since the frequency f is very high, the wavelength xcex=v/f is reduced to a similar size to the micro-cavities and micro-convexities. Acoustic phonons building the surface acoustics wave are scattered by the micro-defects, because phonons are mostly perturbed by the objects of the same size as the wavelength. Besides the high propagation loss xcex94Ep, the defects-rich diamond film decreases the yield of SAW devices by raising the rate of electrode-pattern cutting. Micro-defects prevent diamond SAW devices from growing to practical SAW devices through the large propagation loss xcex94Ep and the low yield. Here xe2x80x9cyieldxe2x80x9d means a ratio of the number of good products to the number of all products.
One purpose of the present invention is to produce diamond SAW devices having low propagation loss. Another purpose of the present invention is to enhance the yield of the diamond SAW devices. In addition to SAW devices, this invention can be applied to raising the yield of the production of microelectronic devices or micromachines making use of small wire patterns with a breadth less than 5.0 xcexcm through reducing the rate of the wiring pattern cutting.
Higher frequency requires SAW filters to make a still better piezoelectric film. The undercoating diamond film must be flat, smooth and defect-free for producing (Diamond)/(piezoelectric material) SAW devices. For this purpose, a simple and reliable estimation technique of diamond films is indispensable besides fabrication of good diamond films. This invention proposes also a method for easily estimating diamond films.
Diamond films have been examined by observing micro-defects, micro-concavities or micro-convexities by scanning electron microscopes or atomic force microscopes.
{circle around (1)} R. Gahlin, A. Alahelisten, S. Jacobson, xe2x80x9cThe effects of compressive stresses on the abrasion of diamond coatingsxe2x80x9d, Wear 196 (1996) 226-233
{circle around (2)} S. K. Choi, D. Y Jung, S. Y Kweon, S. K. Jung, xe2x80x9cSurface characterization of diamond films polished by thermomechanical polishing methodxe2x80x9d, Thin Solid Films 279 (1996) 110-114
Microscope estimation is a straight and reliable method, since diamond surface is directly observed by a microscope. However, the field of vision is too narrow. The observation field is, e.g., about 10 xcexcmxc3x9710 xcexcm for a scanning electron microscope. In general, a SAW device has a size of about 100 xcexcmxc3x97100 xcexcm to 20000xcexcmxc3x9720000 xcexcm. The area of a SAW device is far broader than the field of vision of microscopes. It would take very much time to observe the whole surface of a SAW device by microscopes. Microscope observation cannot be applied to the examination of diamond surfaces in the process of producing diamond SAW devices.
Namely, there is no estimation method available for examining diamond surface on an industrial scale.
Producing diamond SAW devices requires a comprehensive estimation method of examining the entier diamond surface at a stretch. If the diamond film can be easily examined, one can judge whether a piezoelectric film should be deposited on it or not. If the diamond film is bad, the sample should be abandoned without coating with a piezoelectric film. Another purpose of the present invention is to propose a method of estimating physical property (roughness, defects) of a diamond film.
The Inventors of the present invention tried to utilize a laser scanning surface defect detection apparatus for silicon wafers to examine diamond films of SAW devices. Diamond is different from silicon in many physical properties. A laser scanning surface defect. detection apparatus for silicon wafers cannot be readily used for estimating a diamond. Indeed, a laser scanning surface defect detection apparatus has never been used to examine diamond films. FIG. 4 shows a schematic view of the laser scanning surface defect detection apparatus.
The apparatus has an integral sphere, a laser, a lens and a photomultiplier. The inner wall of the integral sphere is a mirror. The integral sphere has an opening at the bottom. The semiconductor laser (L) is mounted at a niche on a side of the integral sphere for emitting an inspecting light beam slantingly down. The semiconductor laser can be replaced by a gas laser, e.g. a Hexe2x80x94Ne laser. The integral sphere has an outlet (U) on the other side for taking out a reflected beam. The inspection comprises the steps of mounting an object wafer on a stage, bringing the bottom opening of the integral sphere close to the wafer (KTW), irradiating a point (T) on the surface of the wafer by the laser beam (LT), and measuring the power of the reflected light (S). The incident beam angle is equal to the refection angle. The laser (L) and the outlet (U) are determined to be symmetric in the integral sphere. Then, ∠LTK=∠UTW If the surface is flat, all the reflected beams go out through the side opening (U). The photomultiplier detects no light. When a defect or a piece of dust lies on the object point (T), a portion of the laser beams is scattered by the defect or dust. The scattering beams impinge upon other parts of the wall of the integral sphere. The photomultiplier senses the scattered beams. FIG. 13 shows a bit of dust lying on the surface of the wafer. Parallel laser beams are randomly reflected by the dust in the directions E, F, G. etc. Reflected beams do not exit through the outlet (U) but impinge upon the inner wall of the integrated sphere and are reflected to the photomultiplier. The photomultiplier measures the scattering light with high sensitivity. The photomultiplier detects the existence of a dust at the sampling point (T) by an increase of its photocurrent. The bigger the dust is, the stronger the scattering light becomes. The reflected light power is in proportion to the size of the dust. It is easy to understand the scattering of light by dust. However, the photomultiplier can also detect geometrical surface defects. FIG. 14 shows a notch, a surface defect, on the surface of the silicon wafer. Silicon has a high refractive index (n=3.5) and high absorption for the visible light emitted from the laser. The complex refractive index n+jk of silicon has a large real part n and a large imaginary part k for the visible light. Laser light is strongly reflected at the interface between air and silicon. The notch (M) scatters the beam in other directions (MH) than that of the outlet (U). The scattered beams are again reflected by the inner wall of the integral sphere and arrive at the photomultiplier for generating a photocurrent. The existence of dust or a defect on the object point of the silicon wafer can easily be detected by the random scattering of light beams.
Since a Si wafer is a wide disc, defects and dust of all the portions are inspected by moving the stage in the two-dimensional XY-plane with regard to the integral sphere. There are two modes of movement of the stage. One is an assembly of rotation and parallel displacement of the shaft. The sampling point (T) moves in a spiral. This mode is called a spiral mode. The other is an assembly of x-direction displacement and y-direction displacement of the wafer. This is called a scanning mode. The overall movement enables the apparatus to detect dust or defects on the whole surface of the silicon wafer.
In the silicon semiconductor industry it is well known to use a laser scanning surface defect detection apparatus. It takes only a short time to examine a pretty wide silicon wafer by the laser scanning surface defect detection apparatus. Unlike observation by a microscope, the apparatus can measure the number or the density of defects or dust quantitatively over a wide range. However, the laser scanning surface defect detection apparatus cannot be readily used to examine the surface of diamond.
Diamond is transparent for the visible light emitted by the laser unlike silicon. Namely, the complex refractive index n+jk has a vanishing imaginary part k=0. The real part n=2.4 is also lower than the real part of silicon. The reflection rate is r=(nxe2x88x921)/(n+1) for the vertical incidence. Since n is small, the reflection rate is also small. Transparency allows the visible light to penetrate into the diamond crystal. Any notch (M) on diamond cannot reflect the laser light in FIG. 3(a). A visible light beam passes a diamond bulk crystal without loss along a light path LMP in FIG. 3(a). If a diamond film is formed on an opaque substrate of a foreign material, the light passes the transparent diamond and arrives at the interface between the diamond film and the substrate. The beam is reflected by the foreign material substrate along the light path LNQ in FIG. 3(a). Since diamond is transparent, the reflection occurs not on the diamond surface but on the interface between the diamond film and the substrate. For this reason, the laser scanning surface defect detection apparatus cannot be applied to the examination of the surface of diamond. The Inventors thought that the laser scanning surface defect detection apparatus is a promising candidate for an inspection apparatus for diamond despite the difficulty of transparency.
One purpose of the present invention is to provide an improvement of the laser scanning surface defect detection apparatus so that it would be suitable for examining surface defects of diamond. Another purpose of the present invention is to provide a diamond substrate which is suitable for use as the substrate of SAW devices by examining surface defects by the laser scanning surface defect detection apparatus. The further purpose of the present invention is to provide a method of selecting diamond substrates suitable to form a piezoelectric film thereupon.
This invention coats a diamond film or substrate with a thin metal or non-metal film having a high reflection rate for the laser light. The thin film enables the laser scanning surface defect detection apparatus to detect defects or dust on a surface of the diamond, since the thin film reflects the laser light. The thin film should be made from a metal or non-metal having a reflectance greater than 50%. The thickness of the coating film is less than 100 nm. The surface of the coating has the same convexities and concavities as the surface of the diamond, since the coating film is thin enough. The coating layer transcribes the diamond surface. If the diamond has a surface defect (M), the coating layer has a similar surface defect (K), as shown in FIG. 3(b). Most of the beams are reflected at the coating layer. The rest penetrating into the coating layer is also reflected at the interface between the coating layer and the diamond. The material of the coating layer is, e.g., aluminum (Al), gold (Au), silver (Ag), copper (Cu), nickel (Ni) or so. Thin film of the metal has a sufficient reflection rate to the laser light. If the diamond has defects or dust, the coating layer has similar anomalies due to the defects or dust at the same spots. The formal anomalies reflect the laser beams at random. Then the laser beams are scattered indirectly by the defects or dust on the diamond. The coating layer enables a conventional laser scanning surface defect detection apparatus to detect defects or dust on an object diamond. Thus, the laser scanning surface defect detection apparatus can now be used for the estimation of diamond surfaces.
FIG. 2 shows a diamond crystal covered by a metal coating layer. The coated diamond is examined with regard to defects or dust by the laser scanning surface defect detection apparatus. If dust lies on the surface, the dust itself scatters the laser light. The scattered light arrives at the photomultiplier. The existence of the dust can be surely detected by the apparatus. However, defect detection is a problem. As shown in FIG. 3(b), the metal coating layer has the same geometric anomaly as the defect, because the metal layer is thin enough. Most of the laser light is reflected at K point. The scattering beam goes along KR. A part of the rest is reflected at M point. The scattered beams KR and MI are again reflected at some portions of the integral sphere and reach the photomultiplier. Thus, the existence of the defect is also be detected by the apparatus. Therefore, the laser scanning surface defect detection apparatus which is inherently directed to examining Si wafers can now be diverted to the examination of defects and dust on diamond surfaces by coating the object diamond with a thin opaque layer.
Diamond films are now examined by the laser scanning surface defect detection apparatus. The Inventors discovered that the critical density is 300 particles/cm2 of the surface defect for diamond films by the examinations. Diamond films which have more than 300 particles/cm2 defects shall be rejected for the substrate of SAW devices. The diamond films with defect density less than 300 particles/cm2 shall be adopted as a substrate for SAW devices. The most important factor is the surface defect density for estimating diamond films as the substrate of SAW devices.
The surface roughness Ra of diamond films has an influence upon the quality of the SAW devices in addition to surface defects. The surface should have a surface roughness lower than Ra 20 nm. In particular, less than Ra 10 nm is preferable for the substrates of SAW devices. The roughness is the second criterion for estimating the quality of diamond films.
Another factor is the size of a diamond-coating wafer. Raising productivity requires a wafer size of more than 2 inches of diameter. Namely, the surface area should be wider than 19 cm2 in the case of a circular wafer.
Inner stress is a further factor for defining a desirable diamond-coated wafer. If a wafer has a big inner stress, the mother materials, silicon or a metal, will be distorted. Distortion of the mother substrate deforms or bends the diamond film either in a convex or in a concave. The deformation of the diamond film reflects the laser light unevenly. A slight slanting of the wafer forbids the reflected light from going out through the outlet U and forces a part of the reflected light into the photomultiplier. Thus, inner stress should be weaker than 1.5xc3x97108 Pa (150 MPa=1500 bar). Besides, the distortion which is defined as the height of the center from the surface containing the periphery should be smaller than xc2x140 xcexcm.
Of course, a pure diamond film is most suitable. But a diamond coated with a thin carbon is also allowable, since the diamond is directed for a substrate of SAW devices. In the case, the carbon overcoat layer should be less than 10 nm in thickness. FIG. 1 shows the following requirements for the diamond film for SAW devices in the present invention.
(1) defect density xe2x89xa6300/cm2 
(2) surface roughness xe2x89xa6Ra10 nm
(3) area xe2x89xa719 cm2 
(4) inner stress xe2x89xa6150 MPa
(5) carbon overcoating xe2x89xa610 nm.
Instead of a film, a freestanding, homogeneous diamond substrate without foreign material substrate is also available. In the full diamond substrate, the thickness of the diamond crystal should be 100 xcexcm to 2000 xcexcm. The complete diamond substrate makes excellent SAW devices.
Is Complex substrates having a foreign material, e.g. silicon, substrate and a diamond film coating the substrate are, in general, made use for the substrate of SAW devices. The mother foreign material should have a thickness between 0 xcexcm and 2000 xcexcm. At the limit of 0 xcexcm, the case coincides with the former faill homogenous diamond substrate. The thickness of the diamond film should be more than 1 xcexcm. The mother substrate and the diamond film should have a roughness less than Ra20 nm. In particular, the surface roughness of the diamond film preferably is less than Ra10 nm.
(6) in the case of a homogeneous full diamond substrate, the thickness should be 100 xcexcm to 2000 xcexcm.
(7) in the case of a diamond/mother complex substrate, the mother substrate thickness should be 0 xcexcm to 2000 xcexcm.
(8) in the case of a diamond/mother complex substrate, the diamond film thickness should be more than 1 xcexcm.
(9) the surface roughness of the diamond film or substrate should be less than Ra20 nm.
Although many proposals have been discussed and proved that diamond SAW devices would be superior in the SAW velocity to other materials, diamond SAW devices are suffering from extremely low yield due to the large propagation loss of the SAW devices having piezoelectric films upon diamonds. The low yield resulted from the lack of the method of a preliminary examination of diamond crystals. Since the property of diamond films could not be measured, SAW devices made on the property-unknown diamond crystals were tested, which lowered the yield and raised the production cost. This invention provides an estimation method which can examine diamond films or bulks at a preparatory step of making diamond SAW devices. This invention makes the best use of the laser scanning surface defect detection apparatus which prevails in silicon semiconductor industries. The laser scanning surface defect detection apparatus cannot be employed for examining diamond surfaces as it is. Preparatory coating of thin films of a metal or another opaque material enables the laser scanning surface defect detection apparatus to examine the defects of diamond surfaces. The examination reveals the density of defects and dust on diamond films or bulks for determining whether piezoelectric films and electrodes should be made upon the diamond samples at a preliminary step. Diamond crystals having defect and dust density of more than 300 particles cmxe2x88x922 cannot form good piezoelectric films and good interdigital transducers thereupon. Higher defect density than 300 particles cmxe2x88x922 lowers the yield by inviting electrode breaking or other default. Diamond surfaces of a defect density lower than 300 particles cmxe2x88x922 pass the examination. The passed diamond enables manufactures to produce low loss SAW filters available for high frequency by depositing piezoelectric films and making interdigital transducers on the diamond surfaces.